Openingup about her relationship with Sean Penn for the first time, actress Charlize Theron tells Vogue that moving from friendship to dating took time. "Sean and I have known each other for 18 years," she told the magazine. "We were just really good friends. And it's been slow going because you are aware that you could screw that up and lose it. We had to think about it. But at the end of the day, it was somewhat effortless. It just kind of naturally happened, and before I knew it, I was in something that was making my life better -- the people who really love me can see the effect it has had on me."